1. Field of the Invention
The present invention relates to an optical recording medium, an optical recording and reproducing apparatus using the optical recording medium, and a method of manufacturing an optical recording medium. In this case, the optical recording medium and the optical recording and reproducing apparatus designate an optical recording medium and an optical recording and reproducing apparatus to which at least any of the optical recording or reproducing is carried out, respectively. Further, the optical recording or reproducing is performed by using a laser light and a high density recording is particularly intended.
2. Description of the Related Art
When at least any of an optical recording or reproducing (hereinafter referred to as recording and reproducing simply) is performed for an optical recording medium by using a laser light, if it is assumed that the wavelength of the laser light is taken as .lambda. and the numerical aperture of a lens system, namely an objective lens of a recording and reproducing apparatus is taken as N.A., a spot diameter .0. of recording and reproducing laser light is given by the following expression (1). EQU .0.=1.22 .lambda./(N.A.)
Accordingly, if it is intended that a high density is made in an optical recording medium, e.g., an optical disk, it is effective that .lambda. is set small, that is, the wavelength is set short and the N.A. is set high.
As to a tolerance amount for a scattering of mechanical characteristics in the recording and reproducing apparatus, if the thickness of a light transmission layer on the laser light incident side of the optical recording medium is assumed as t, it is known that the following relations are established. EQU Focal Depth FD=.lambda./(N.A.).sup.2 (2)
Tolerance (margin) of inclination (skew) of the optical recording medium EQU SM.varies..lambda./(N.A.).sup.3 /t (3)
Margin of thickness t of light transmission layer EQU TM.varies..lambda./(N.A.).sup.4 (4)
Since in the recording and reproducing apparatus of a conventional CD ( compact disk), for example, .lambda.=0.78 .mu.m, N.A.=0.45 and from the above relation expressions, the followings are obtained
.0.=2.11 .mu.m PA0 FD=3.85 .mu.m PA0 SM=8.56 .alpha./t (.alpha. is an arbitrary constant) PA0 TM=19.02 .beta. (.beta. is an arbitrary constant) PA0 .0.=1.32 .mu.m PA0 FD=1.81 .mu.m PA0 SM=3.01 .alpha./t PA0 TM=5.02 .beta. PA0 .0.=0.93 .mu.m PA0 FD=0.90 .mu.m PA0 SM=1.06 .alpha./t PA0 TM=5.02 .beta.
Because in the recording and reproducing apparatus of a DVD (digital video disk) .lambda.=0.65 .mu.m, N.A.=0.6 and from the above relation expressions, the followings are obtained:
When respective values of the CD and the DVD are compared each other, the SM of DVD is about 1/3 time of the CD and hence it is necessary that the skew of DVD is suppressed to 1/3 for the conventional CD.
In fact, the skew standard of CD is 0.6.degree. so that it is almost impossible to suppress the same to 0.2.degree..
Therefore, in order to correct the same, the thickness t of the light transmission layer is made to be 0.6 mm or an half of that of CD, whereby the skew margin SM is doubled to be the standard of 0.4.degree..
With respect to the margin of thickness irregularity, since the DVD is about 1/4 or less for the CD, in comparison with the fact that the error of thickness of transparent plastic substrate corresponding to the light transmission layer on the laser incident side of the CD is set to be 100 .mu.m, it is suppressed to be 30 .mu.m in the DVD. This value can be sufficiently manufactured by the conventional molding technique for the substrate.
As to the FD, the FD of DVD is about 1/2 of that of CD. It is also possible that a target information layer can be set within the focal depth by the conventional molding technique and bonding technique upon manufacturing a DVD.
However, since it is desired that the recording capacity is further increased from the recording capacity of 4.7 GB (giga byte) of DVD to improve the recording density, it is required that the N.A. is further increased from that of a conventional DVD. About this requirement, as proposed by Patent Application No. Hei 9-185130 by the same applicant of the present application, a high N.A. is proposed by a 2-group lens. In this case, it is confirmed that the N.A. can be made 0.9 higher than 0.7. Then, an optical parameter of this case is N.A./.lambda..gtoreq.1.2 [.mu.m.sup.-1 ], .lambda..ltoreq.0.68 .mu.m and a thickness irregularity .DELTA.t of light transmission layer is set within .+-.5.26 .lambda./(N.A.).sup.4.mu.m. If N.A.=0.85, for example, this is 2 times of N.A.=0.6. Therefore, even if a short wavelength laser light source is not used, the surface density can be made 2 times. Thus, if the modulation system same as that of DVD is employed, there is realized an optical recording disk, that is, an optical recording medium of 4.7.times.2=9.4 GB.
In this case, however, each of the margins mentioned above is decreased. That is, if .lambda.=0.64 .mu.m is established, the following expressions are obtained from the above expressions (1) to (4):
In this way, when for the sake of presenting a high density, the N.A. is increased up to 0.85 as described above, for example, its SM is lowered by about 1/3 as compared with the DVD, its TM becomes about 1/5 and its FD becomes about 1/2 or margins become severe. Therefore, it becomes impossible to realize an optical recording medium with the structure same as that of the DVD as well as the conventional DC.
Further, if a laser light source with a wavelength of 400 nm or less than that is used, each of the above-mentioned margins is decreased further.